1. Field of the Invention
The present invention relates to crystalline catalyst compositions for the conversion of hydrocarbons, particularly for hydrocracking, and a process for the preparation of such crystalline catalyst compositions.
The term "crystalline catalyst composition" as used herein, as will be apparent from the explanation hereafter, does not mean that the whole catalyst is in crystalline form, but means that a substantial proportion of the zeolite component in the catalyst is present while retaining the crystal structure of zeolite. The terms "Group VI metal" and "Group VIII metal" as used herein mean "Group VI metal of the Periodic Table" and "Group VIII metal of the Periodic Table", respectively.
2. Development of the Invention
In general, hydrocarbon hydrocracking catalysts have two functions: acid activity to cleave a carbon-carbon bond and hydrogenation activity to hydrogenate olefinic molecules resulting from the cleavage. The acid activity is exhibited by acid sites of the catalyst and the hydrogenation activity is exhibited by metals deposited on the catalyst. Crystalline aluminosilicate zeolite (hereinafter sometimes referred to merely as "zeolite") possesses a crystal structure in which silicon and aluminum are regularly bond. Thus, the density of the acid sites of the zeolite, the acid sites occurring at the contact point between two elements, is much greater than those of inorganic compounds such as silica and alumina. For this reason, zeolite is often used as one component of this type of catalyst.
Zeolite, however, has the disadvantage in that upon exposure to water vapor at elevated temperatures, its crystal structure is easily destroyed, resulting in a reduction in catalytic activity. In order to stabilize the crystal structure of zeolite against water vapor at elevated temperatures, various improvements have been made. For example, U.S. Pat. Nos. 3,536,606, 3,867,277 and 4,036,739 disclose a method of preparing stabilized and highly active zeolite, in which sodium ions contained in zeolite are partially replaced by ammonium ions, the zeolite is calcined in a steam atmosphere while controlling the temperature, processing time and steam partial pressure, sodium ions remaining in the zeolite are further replaced with ammonium ions, and then the zeolite is calcined. U.S. Pat. No. 3,669,873 discloses a method of preparing stabilized and highly active zeolite by subjecting the same to ion exchange using alkaline earth metal ions or rare earth metal ions.
These methods enable one to improve the tolerance of zeolite to water vapor at elevated temperature, i.e., the hydrothermal stability of zeolite. However, in employing these methods, another problem is encountered, that is, when certain metal components such as Group VI metal components of Group VI metals of the Periodic Table are deposited on zeolite, the crystal structure is destroyed and, therefore, adequate catalytic activity cannot be obtained.
As discussed above, hydrocracking catalysts are composed of two components: (1) a component having acid sites, such as zeolite, and (2) a metal component having hydrogenation activity deposited on component (1). As component (2), metal components of Group VI and VIII metals of the Periodic Table are used. In particular, when metal components of Group VI metals are deposited, there is a tendency for the crystal structure to be destroyed which leads to a serious reduction in the number of zeolite acid sites and thus a high activity hydrocracking catalyst cannot be obtained.
Metal component (2) can be deposited by various procedures such as dipping a support in a solution containing the metal component, kneading a support with a solution containing the metal component and depositing the metal component on a support by ion exchange. With all of these methods there is observed the above-described tendency that the crystal structure of zeolite is destroyed.
The methods disclosed in the above-described U.S. Patent specifications have some additional disadvantages. For example, the methods are disadvantageous for industrial use since they are complicated operation, and, even if ion exchange using alkaline earth metal ions and the like is applied, hydrothermal stability is improved insufficiently.
U.S. Pat. No. 3,706,693 and Canadian Pat. Nos. 972,308 and 972,340 disclose hydrocarbon conversion catalysts which are prepared by contacting a support comprising zeolite and inorganic oxides such as alumina with an aqueous solution containing Group VI metal compounds, Group VIII metal compounds and acids of phosphorus, the weight ratio of phosphorus to the Group VI metal compound (calculated as an oxide) being from 0.05:1 to 0.5:1, and having an initial pH value of below 3.0. It is described that although the crystal structure of zeolite is destroyed since a large amount of phosphoric acid is added under strongly acid conditions in depositing the hydrogenation metal component, hydrocracking activity, denitrogenation activity and selectivity for an intermediate fraction can be increased. In accordance with the methods described in the above-described U.S. and Canadian Patent specifications, the high cracking activity of zeolite is intentionally decreased by destroying the crystal structure of zeolite and, therefore, the hydrocracking activity is not sufficiently high.